Phosphate coating method



States nite 3,66%,066 PHOSITE CGATILJG METHOD Wilford H. Ross, In, St. Paul, Minn, and James E. Ma-

loney, Oak Park, and John M. Stapleton, Detroit, Mich, assignors to Detrex Chemical Industries, Inc., Detroit, Mich, a corporation of Michigan No Drawing. Filed May 18, 1961, Ser. No. 110,851 2 Claims. (til. 1486.15)

This invention relates to improved phosphate coating compositions and a method for producing phosphate coatings on metallic articles. More particularly it relates to an improved phosphate coating composition for use on ferrous metal surfaces, which is suitable for use with hard or alkaline water over a wide pH range, whereby aniron phosphate coating is formed on the ferrous surface.

Compositions useful for the application of phosphate coatings to metals are commonly classified under three different systems. One system of classification is based on the presence or absence in the solution of metal ions which enter into the coating applied. This system results in the designations coating and non-coating phosphates. A coating phosphate is a phosphate composition which contains ions such as zinc, manganese, iron, chromium and the like in the p'hosphating solution, which ions are deposited on the surface of the metal being coated together with phosphate ions. This classification ignores reference to the metal being coated. A non-coating phosphate is one which does not contain ions which form part of the coating applied. The metal ions in a phosphate coating of this type are supplied by the metal being coated.

A second system of classification depends upon the composition of the phosphate coating formed. Thus, a phosphate composition in solution which yields a coating consisting principally of iron phosphate is described as an iron phosphate solution, even though the solution itself be free of iron.

A third system of classification depends on both the metal coated and the composition of the coating. Thus, a phosphate solution which deposits a coating consisting principally of iron phosphate on a ferrous metalsurface is referred to as an iron-on-iron phosphate solution. Similarly, a phosphate coating solution which deposits a coating consisting principally of zinc phosphate on a ferrous metal surface is referred to as a zinc-on-iron phosphate solution.

The phosphating compositions produced in accordance with the present invention, as classified under the above three systems, are non-coating, iron and iron-oniron, respectively. Compositions of this type are conventionally applied by spraying solutions thereof on the article to be treated.

One of the difficulties which have long been experienced with phosphating solutions containing compositions of the above type is a gradual rise in pH caused by the carryover of basic constituents from prior alkaline cleaning stages, and also 'by the replenishment of lost water with hard or alkaline water. This difticulty has been accentuated by the comparatively narrow eifective operating pH range of the conventional solutions of this type.

Attempts have been made in the past to meet these problems by the use in such compositions of polyphosphate compounds, such as pyrophosph ates and the like. These attempts have not been very successful because the polyphosphate compounds hydrolyze rapidly to the corresponding orthophosphate compound. This hydrolysis is accelerated by higher temperatures and by acid solutions. In U.S. Patent 2,067,007, for example, it is disclosed that zinc pyrophosphate may he used to prevent hydrolysis of the pyrophosphate to orthophosphate. However, when zinc pyrophosph-ate is used in this manner, the solution, in

order to prevent hydrolysis must also be used at ordinary temperatures, which then necessitates either a very high degree of acceleration or excessively long contact times. Therefore, oxidizing agents and metals are normally used to accelerate the action of the phosphating solution sufiiciently to effect satisfactory coating of iron or steel.

Furthermore, in U.S. Patent 2,856,322, it is disclosed that the presence of orthophosphate ions in a pyrophosphate coating solution is deleterious, i.e., the solution should be kept free of the products of the hydrolysis of pyro-phosph-ate ions.

Similarly, in U.S. Patent 2,891,884, it is disclosed that the presence of an orthophosphate in a pyrop'hosphate coating solution should be avoided, particularly when the ratio of pyrophosphate to orthophosphate drops below 2.5: 1. The pH in this case should be 2.7 to 3.7, no coating at all being produced at a pH of 4.2 and higher. Additionally, an oxidizing agent, sodium nitrite, is required as an accelerator.

It is therefore an object of this invention to provide improved phosphate coating compositions of the type defined above and a method for applying the compositions wherein relatively high temperatures may be used.

It is another object of this invention to provide such improved compositions that may be utilized in the presence of a hard or alkaline water diluent.

It is another object of this invention to provide such improved compositions that are effective without the addition of accelerators such as oxidizing agents and heavy metals.

It is another object of this invention to provide such improved compositions that effectively utilize a combination of orthophosphate and pyrophosphate compounds.

It is another object of this invention to provide such improved compositions which will retain their coating capacity over relatively long periods of time without makeup.

These and other objects have now been accomplished by the discovery that relatively large proportions of orthophosphate compounds can be used with pyrophosphate compounds in the presence of an amount of a silicofluoride compound to give a strongly buffered coating solution which coats ferrous metal surfaces satisfactorily with an integral adherent iron phosphate coating, within a pH range of from about 3.8 to about 5.2, and at relatively high temperatures of more than about F.

xamples of compositions suitable for use in accordance with this invention preferably have an analysis falling within the following ranges (parts being expressed as percent by weight of a dry composition) Percent by weight Sodium dihydrogen orthophosphate 40-60 Sodium di-hydrogen pyrophospha-te 57-10 Sodium silicofluoride 3-10 The corresponding potassium salts of orthophosphoric and pyrophosphoric acid are equivalent in all respects to the herein disclosed compounds. Also, if desired, a total of from about 0.1% to about 10% of the total weight of such dry composition may be formed with a conventional nonionic detergent, and the phosphate coating may be deliberately colored by the incorporation into the dry composition small quantities of coloring agents, such as, for example, sodium molybdate where a bluish coating is desired.

The dry composition is dissolved in water to give concentrations of from about 1 to about 5 ounces per gallon of water, and preferably about 2 ounces per gallon of water, thereby producing a coating solution having a pH in the range of from about 3.8 to about 5.2. The desired phosphate coating is then produced by spraying a ferrous 3 metal surface with this solution at a temperature in the range of from about 140 F .to about 180 F., and preferably at about 160 F. A coating is normally effected in about 1 to 2 minutes.

The following specific example illustrates the use of the improved compositions and method of this invention:

Example 1 A dry mix having the following composition, parts being expressed as percent by weight, is prepared:

Percent by weight Sodium dihydrogen orthophosphate 49.85 Sodium dihydrogen pyrophosphate 40.00 Sodium silicofiuoride 5.00 Non-ionic detergent 5.00 Sodium molybdate (coloring agent) 0.15

Total 100.00

formed on the surface thereof in amounts of upward of i mgm. per ft. The article is then rinsed with water and dried, the coated article having a bluish color.

During the treatment in accordance with the process of this invention, the solution containing the compositions of this invention preferably initially has a pH in the range of from about 3.8 to about 4.2. Such a solution will then continue to coat satisfactorily until its pH reaches about 5.2.

The following examples are illustrative of further specific dry compositions which when utilized in accordance with Example 1 are effective to similarly coat ferrous metal surfaces, parts being expressed as percent by weight:

Example 2 Percent by weight Sodium dihydrogen orthophosphate 40.00 Sodium dihydrogen pyrophosphate 50.00 Sodium silicofluoride 10.00

Total 100.00

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of 3.7, and when so dissolved in soft water has a pH of 3.3.

Example 3 Percent by weight Sodium dihydrogen orthophosphate 40.00 Sodium dihydrogen pyrophosphate 57.00 Sodium silicofiuoride 3.00

Total 100.00

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of Water has a pH of 4.1, and when so dissolved in soft water has a pH of 3.7.

Example 5 Percent by weight Sodium dihydrogen orthophosphate 50.00 Sodium dihydrogen pyrophosphate 40.00 Sodium silicofluoride 5.00 (Non-ionic detergent) 5.00

Total 100.00

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of 4.2, and when so dissolved in soft water has a pH of 3.8.

Example 6 Percent by weight Sodium dihydrogen orthophosphate 52.00 Sodium dihydrogen pyrophosphate 42.00 Sodium silicofiuoride 5.90 (Non-ionic detergent) 0.10

Total 100.00

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of 4.1, and when so dissolved in soft water has a pH of 3.7.

Example 7 Percent by weight Sodium dihydrogen orthophosphate 52.60 Sodium dihydrogen pyrophosphate 42.10 Sodium silicofluoride 5.30

Total 100.00

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of 4.2, and when so dissolved in soft water has a pH of 3.8.

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of 3.9, and when so dissolved in soft water has a pH of 3.5.

Example 9 Percent by weight Sodium dihydrogen orthophosphate 60.00 Sodium dihydrogen pyrophosphate 37.00 Sodium silicofiuoride 3.00

Total 100.00

This composition when dissolved in alkaline water at a concentration of 2 ounces per gallon of water has a pH of 4.4, and when so dissolved in soft water has a pH of 4.0.

Example 10 Percent by weight Sodium dihydrogen orthophosphate 75.00 Sodium dihydrogen pyrophosphate 10.00 Sodium silicofluoride 10.00 (Non-ionic detergent) 5.00

Total 100.00

This composition when dissolved in alkaline Water at a concentration of 2 ounces per gallon of water has a pH of 3.7, and when so dissolved in soft water has a pH of 3.3.

The solutions prepared from the dry compositions of Examples 29 in accordance with Example 1, will coat satisfactorily in a temperature range of from about F. to about F., at concentrations of in the range of from about 1 ounce per gallon to about 5 ounces per gallon and with spray times of from about one to two minutes.

Having thus described our invention we claim:

1. A process for treating the surface of a ferrous metal article which comprises the step of contacting said surface with an aqueous solution consisting essentially of (on a dry weight basis):

Parts by weight Alkali metal dihydrogen orthophosphate 40 to '60 Alkali metal dihydrogen pyrophosphate 57 to 10 Alkali metal silicofluoride 3 to 10 said dry ingredients being dissolved in water at a concentration in the range of from about 1 to about 5 ounces per gallon of water, said solution having a pH in the range of from about 3.8 to about 5.2, While maintaining the temperature of said solution in the range of from about 140 F. to about 180 F.

2. The process in accordance with claim 1 wherein the alkali metal is selected from the group consisting of sodium and potassium.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,007 Darsey J an. 5, 1937 2,312,855 Thompson Mar. 2, 1943 2,891,884 'Rausch et a1 June 23, 1959 

1. A PROCESS FOR TREATING THE SURFACE OF A FERROUS METAL ARTICLE WHICH COMPRISES THE STEP OF CONTACTING SAID SURFACE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF (ON A DRY WEIGHT BASIS): 